1. Field of Invention
The present invention relates to a rolling mill provided with a pair of work rolls having different diameters, and a sheet rolling method employing the same rolling mill.
2. Description of Related Art
A conventional rolling mill is provided with upper and lower work rolls respectively having different diameters and supported by upper and lower backup rolls. In this type of rolling mill, the larger work roll, i.e., the work roll having a larger diameter, is driven by a motor or the like to roll a sheet. A rolling mill provided with work rolls having different diameters, sometimes called a differential rolling mill (as compared with ordinary rolling mills provided with work rolls of the same diameter) is able to roll a sheet at a high draft by a low rolling force, which is advantageous in manufacturing steel sheets by rolling. Since only a small rolling force is necessary, edge drop resulting from the flattening of the rolls can be suppressed and hence steel sheets having a small thickness deviation can be manufactured.
Generally, as shown in FIG. 8, working rolls 11xe2x80x2 and 12xe2x80x2 included in most rolling mills are shifted downstream by an offset e with respect to backup rolls 13xe2x80x2 and 14xe2x80x2. The work rolls are thus shifted downstream with respect to the backup rolls because a rolling mill in which work rolls are shifted downstream with respect to backup rolls is able to stabilize loading conditions for loading a rolled sheet more effectively than a rolling mill in which work rolls are shifted upstream with respect to backup rolls.
A related art is disclosed in JP-B No. 47421/1976.
Recently, hot rolling techniques for hot-rolling sheets are required to be capable of rolling sheets in a greater rolling width, i.e., the width of the rolled sheet, and in smaller thickness, and of rolling sheets at higher drafts. However, the diameter of the smaller work roll of the differential rolling mill is smaller and the mechanical strength of the smaller work roll is insufficient to meet the foregoing requirements. More specifically, a high stress is induced in necks, including stepped parts, at the joints of the body, which is used for rolling, of the smaller work roller and the journals, supported in bearings, of the smaller work roll.
Thus, the upper limit of the rolling width of steel sheets hot-rolled by differential rolling mills has been 4 ft (about 1200 mm). Even the differential rolling mill that needs a relatively low rolling force requires a high rolling force exceeding 3000 tons (3000 tf=2.94xc3x97107 N) when the rolling width exceeds 4 ft. An excessively high stress unbearable by the mechanical strength of the smaller work roll is thus induced in the necks of the smaller work roll.
The present invention is intended to meet the foregoing requirements required of rolling mills for hot-rolling sheets, including capability of rolling sheets in an increased width exceeding 4 ft by reducing mechanical load on work rolls.
A sheet rolling method according to a first aspect of the present invention includes: disposing a pair of work rolls respectively having different diameters between upper and lower backup rolls; and driving only the large-diameter work roll having the greater diameter for rolling to produce a sheet; wherein the small-diameter work roll having the smaller diameter is disposed so that a rotational axis of the small-diameter work roll is positioned on a mill center or a downstream side with respect to the mill center in a rolling direction, and the large-diameter work roll is disposed so that a rotational axis of the large-diameter work roll is positioned on a downstream side with respect to the rotational axis of the small-diameter work roll in the rolling direction.
Since the sheet rolling method does not shift both the two working rolls on the upstream side of the mill center plane including the center axes of the backup rolls with respect to the rolling direction, loading conditions for rolling the sheet is stabilized, and the sheet can be smoothly and continuously rolled.
This sheet rolling method is characterized in reducing mechanical load on the work rolls even when a high rolling force is necessary for rolling a wide sheet.
The ability of the sheet rolling method to reduce the mechanical load on the work rolls can be reasoned as follows.
When a rolling mill provided with two work rolls respectively having different diameters operates for rolling to produce a sheet, the following forces a) to c) are exerted on the journals, supported in bearings, of the smaller work roll having a smaller diameter:
a) A horizontal force acting downstream with respect to the rolling direction resulting from driving only the large-diameter work roll having the greater diameter and exerted on the small-diameter work roll by a sheet being rolled (Force SR1 in FIG. 3);
b) A roll bender force acting on the work roll in a plane (vertical plane) perpendicular to the rolling direction (Force PB, not shown); and
c) A horizontal force equal to the difference between the horizontal components of vertical forces exerted on the small-diameter work roll by the backup roll and the large-diameter work roll (SB1 and SD1 shown in FIG. 2) (Force Pmt shown in FIG. 2).
These forces are exerted on the journals supported in the bearings to induce stresses in the necks of the small-diameter work roll.
Although all those forces are produced necessarily during the rolling operation, the magnitude (and the direction, in some cases) of the horizontal force (Pmt) produced by the forces (SB1 and SD1) is dependent on the dispositions of the large and the small-diameter work rolls relative to the backup rolls, represented by offsets.
According to an exemplary embodiment of the invention, the small-diameter and the large-diameter work rolls are disposed such that the offset of the axis of the large-diameter work roll with respect to the mill center plane is greater than the offset which could be zero in some cases of the small-diameter work roll with respect to the mill center plane. This arrangement provides for the horizontal component (SB1) of the vertical force exerted by the large-diameter work roll on the small-diameter work roll and used to determine the horizontal force (Pmt, the force c)) to be directed upstream with respect to the rolling direction. Consequently, the horizontal force (Pmt, the force c)) is reduced. Since the direction of the horizontal component (SB1) is opposite to the rolling direction, the horizontal force that acts on the small-diameter work roll, i.e., the resultant force acting on the small-diameter work roll, i.e., the sum of the horizontal force (SR1, the force a)) and the horizontal component (SB1), is reduced. When the horizontal force is reduced, the mechanical load on the small-diameter work roll is reduced accordingly even if the vertical force, such as the force b), does not change. Consequently, a sheet having a big width and a small thickness can be produced and draft at which the sheet can be rolled by one rolling mill can be increased.
In the sheet rolling method according to the first aspect of the present invention, it is preferable that an offset e1 by which the rotational axis of the small-diameter work roll is shifted from the mill center plane, and an offset e2 by which the rotational axis of the large-diameter work roll is shifted from the rotational axis of the small-diameter work roll (refer to FIG. 1 for e1 and e2) meet inequalities:
0 mmxe2x89xa6e1 and 0 mm less than e2 less than 7 mm
when the small-diameter work roll has necks of a diameter of about 270 mm or below, and a rolling force of 3000 tons (3000 tf=2.94xc3x97107 N, 1 ton=1 tf=9800 N) or above is used for rolling work.
The diameter of the body of the small-diameter work roll having the necks of a diameter of 270 mm or below is limited by the relation of the small-diameter work roll with support means including bearings and is considerably small, such as about 400 mm or below. Since the small-diameter work roll has such a small diameter, the sheet can be rolled at a high draft by using a low rolling force. Consequently, edge drop in the sheet can be suppressed and advantages specific to differential rolling mills can be fully utilized.
As mentioned above, it is generally advantageous that both the small-diameter and the large-diameter work roll are shifted downstream of the rolling direction with respect to the mill center plane and that the offset of the large-diameter work roll is greater than the offset of the small-diameter work roll, namely,
0xe2x89xa6e1 and 0 less than e2,
when a high rolling force is used.
However, since the smaller the diameter of the small-diameter work roll, the more effective are the advantages specific to differential rolling mills, and the higher the rolling force, the greater the possibility of increasing rolling width. Accordingly, the offsets e1 and e2 must satisfy inequalities:
0xe2x89xa6e1 and 0 mm less than e2 less than 7 mm
under rolling conditions, where the diameter of the necks of the small-diameter work roll is 270 mm or below and the rolling force is about 3000 tons or above (a proper roll bender force is also added) because the horizontal force of c) increases and a stress excessively high to the small-diameter work roll of a general material is induced in the necks of the small-diameter work roll when the diameter of the necks of the small-diameter work roll and the rolling force meet the foregoing conditions, if e2xe2x89xa60. An undesirable warping of the sheet called bowing, i.e., the upward warping of the leading edge of the sheet passed between the small-diameter and the large-diameter work roll, occurs if 7 mmxe2x89xa6e2. A sheet of a width (rolling width) on the order of 5 ft can be produced by hot-rolling a steel sheet when the rolling force is about 3000 tons or above.
A rolling mill according to a second aspect of the present invention includes: upper and lower backup rolls; and a pair of work rolls respectively having different diameters and disposed between the upper and the lower backup roll; wherein only the large-diameter work roll having the greater diameter is connected to a driving source, and wherein the small-diameter work roll having the smaller diameter is disposed so that a rotational axis of the small-diameter work roll is positioned on a mill center or a downstream side with respect to the mill center in a rolling direction, and the large-diameter work roll is disposed so that a rotational axis of the large-diameter is positioned on a downstream side with respect to the rotational axis of the small-diameter work roll in the rolling direction.
In the rolling mill according to the second aspect of the present invention, it is preferable that the small-diameter work roll has necks of a diameter of 270 mm or below, and an offset e1 by which the axis of the small-diameter work roll is shifted from the mill center plane, and an offset e2 by which the axis of the large-diameter work roll is shifted from the axis of the small-diameter work roll meet inequalities:
0 mmxe2x89xa6e1 and 0 mm less than e2 less than 7 mm.
Since the rolling mill is provided with the small-diameter work roll having the necks of a diameter of about 270 mm or below and a body of a considerably small diameter on the order of, for example, 400 mm, the rolling mill has characteristics specific to differential rolling mills and is capable of rolling a sheet at a high draft. Thus, the rolling mill is capable of producing steel sheets having uniform thickness effectively by rolling.
The rolling mill, in which the respective offsets e1 and e2 of the small-diameter and the large-diameter work roll satisfy the equalities:
0xe2x89xa6e1 and 0 less than e2 less than 7 mm,
has excellent abilities a) to hot-roll a steel sheet having a width (rolling width) on the order of 5 ft by using a rolling force of 3000 tons or above, b) to limit stress induced in the necks of the small-diameter work roll below a level that is not dangerous to the small-diameter work roll formed of a general material even if such a high rolling force is used, and c) to prevent undesirable bowing of the sheet passed between the work rolls.
Preferably, in the sheet rolling mill according to the second aspect of the present invention, the small-diameter work roll has a core formed of a material having a tensile strength of 45 kgf/mm2 or above (4.41xc3x97108 Pa), such as a nickel grain roll (cast high-alloy steel grain roll), a high-chromium alloy roll (high-chromium cast steel), high-speed steel roll (high-speed tool steel) or a forged high-speed steel roll.
When the small-diameter work roll is a nickel grain roll, a high-chromium alloy roll, a high-speed steel roll or a forged high-speed steel roll formed of a material having a tensile strength of 45 kgf/mm2 or above, the rolling method according to the first aspect of the present invention can be advantageously carried out without being subject to restrictions, because a rolling force of abut 3000 tons or above can be exerted on the small-diameter work roll having the necks of a diameter of about 270 mm or above, and the small-diameter work roll formed of a material having a tensile strength of 45 kgf/mm2 or above, which is higher than a maximum stress of about 40 kgf/mm2 (3.92xc3x97108 Pa) that is expected to be induced in the small-diameter work roll when a roll bender force, which is comparatively low because the small-diameter work roll has a small diameter, does not have any problem in mechanical strength. Generally, high-speed steel rolls or forged high-speed steel rolls have a tensile strength of 80 kgf/mm2 (7.84xc3x97108 Pa) or above and hence problems attributable to fatigue resulting from rotation involving repeated stress cycles can be easily avoided.